Trisomy 13

As far as we know, nothing either parent does (or doesn't do) before or during pregnancy can cause a fetus or child to have a chromosome abnormality. Chromosome abnormalities most often occur as a result of a random error when an egg or sperm cell develops or after fertilization; less commonly, chromosome abnormalities may be inherited. Many parents who learn that their fetus or child has a chromosome abnormality worry that X-ray exposure, medications taken before or during pregnancy, or dietary regimes in either the mother or father may have caused the abnormality. Evidence has supported that there is nothing a parent can or cannot do to cause or prevent chromosome abnormalities. Research studies have looked at many different factors, including those mentioned above as well as chemical exposures, geographic location, and ethnicity. Up to this point in time, none of these studies have shown that these factors cause or contribute to chromosome abnormalities.^[1]

Most cases of trisomy 13 are caused by random events during the formation of eggs or sperm in healthy parents (prior to conception). Trisomy 13 is typically due to having three full copies of chromosome 13 in each cell in the body, instead of the usual two copies. This is referred to as complete trisomy 13 or full trisomy 13. The extra genetic material present disrupts the normal course of development, causing the characteristic features of the disorder.

Trisomy 13 can also occur when part of chromosome 13 becomes attached (translocated) to another chromosome during the formation of eggs or sperm, or very early in fetal development. This is referred to as translocation trisomy 13. People with this type of trisomy 13 have the normal two normal copies of chromosome 13, plus an extra copy of chromosome 13 that is attached to another chromosome. Translocation trisomy 13 can be inherited. Approximately 20% of cases of trisomy 13 are caused by translocation trisomy 13. 

In rare cases, only part of chromosome 13 is present in three copies in each cell (rather than the full chromosome); this is called partial trisomy 13. In other rare cases, a person has an extra copy of chromosome 13 in only some of the body's cells; this is called mosaic trisomy 13. The severity of mosaic trisomy 13 depends on the type and proportion of cells that have the extra chromosome.^[2][3]